The invention concerns the mounting and fitting of a printing plate on a plate cylinder of an offset printing machine.
The offset printing technique uses blocks having the shape of a thin metal plate, for instance of aluminum, on which the printing motif is engraved. This engraved plate called a block or a printing plate is fitted on the periphery of a rotary cylinder called a plate cylinder.
For this purpose, such a cylinder is generally provided with a peripheral groove of an appropriate radial depth and arranged parallel to the cylinder axis. The printing plate which may be wrapped all around the cylinder or else only around part of the cylinder has at both ends, with the plate viewed in a wrap-around direction, a bent part destined to be hooked into the cylinder peripheral groove.
Such a fitting method is described for instance in the U.S. Pat. No. No. 4,214,530 patent according to which each bent part has a allowing it to be adapted to, or firmly hooked onto, an edge of a radial wall of a groove cut into the peripheral surface of the cylinder. For arranging or mounting the printing plate, the reference teaches to initially hook the bent part of a first plate end onto the corresponding groove edge and then to wrap the plate around the cylinder in order to hook the other end in the proximity of the second groove edge with which the second part will then be engaged, the distance between the two bent parts being such that only force, i.e., tension, exerted on the plate will enable both bent parts to extend to and exceed the corresponding edge for being ultimately hooked thereon. Obviously, such a fitting system might have the shortcoming of a slight imprecision of distance occurring between the two bent parts of the plate. In cases where this distance is too short, such an error is likely to render the placing of the plate difficult on the cylinder because of excessive tension being required. In the event of an excessive distance, the plate may be retained onto the cylinder too loosely and such an arrangement may fail to keep the plate fixed on the cylinder during the printing process.
Another fitting method currently used consists in using printing plates having an oversize with regard to the peripheral cylinder dimension and having a connection of at least one of its ends to a cam-type locking system for tightening the plate in the rotary direction of the cylinder. Examples of such a design are described in the U.S. Pat. No. 3,626,848 and U.S. Pat. No. 3,757,691 patents. However, the incorporation of such a cam-type locking system into a cylinder groove like the one described herein would involve a difficult and expensive realization as well as the use of a rather broad groove with a resultant inking dead zone.